NTRK1 Protein

NTRK1 Protein

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">NTRK1 Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>Tropomyosin Receptor Kinase A (TrkA)</td>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>NTRK1</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>[P04629](https://www.uniprot.org/uniprot/P04629)</td>
</tr>
<tr>
<td class="label">Alternative Names</td>
<td>TrkA, Trk, p140-NTRK1</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~87 kDa (full-length)</td>
</tr>
<tr>
<td class="label">Protein Length</td>
<td>796 amino acids</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>1q21-q22</td>
</tr>
<tr>
<td class="label">Subcellular Location</td>
<td>Plasma membrane, endosomes, nucleus</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>Trk (Tropomyosin Receptor Kinase) family</td>
</tr>
<tr>
<td class="label">Ligand</td>
<td>Nerve Growth Factor (NGF)</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Candidate</td>
</tr>
<tr>
<td class="label">Small molecule agonists</td>
<td>Various</td>
</tr>
<tr>
<td class="label">Peptide agonists</td>
<td>YEP series</td>
</tr>
<tr>
<td class="label">Gene therapy (AAV)</td>
<td>AAV2-NGF</td>
</tr>
<tr>
<td class="label">Cell therapy</td>
<td>NGF-secreting cells</td>
</tr>
<tr>
<td class="label">Allosteric modulators</td>

NTRK1 Protein

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">NTRK1 Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>Tropomyosin Receptor Kinase A (TrkA)</td>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>NTRK1</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>[P04629](https://www.uniprot.org/uniprot/P04629)</td>
</tr>
<tr>
<td class="label">Alternative Names</td>
<td>TrkA, Trk, p140-NTRK1</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~87 kDa (full-length)</td>
</tr>
<tr>
<td class="label">Protein Length</td>
<td>796 amino acids</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>1q21-q22</td>
</tr>
<tr>
<td class="label">Subcellular Location</td>
<td>Plasma membrane, endosomes, nucleus</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>Trk (Tropomyosin Receptor Kinase) family</td>
</tr>
<tr>
<td class="label">Ligand</td>
<td>Nerve Growth Factor (NGF)</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Candidate</td>
</tr>
<tr>
<td class="label">Small molecule agonists</td>
<td>Various</td>
</tr>
<tr>
<td class="label">Peptide agonists</td>
<td>YEP series</td>
</tr>
<tr>
<td class="label">Gene therapy (AAV)</td>
<td>AAV2-NGF</td>
</tr>
<tr>
<td class="label">Cell therapy</td>
<td>NGF-secreting cells</td>
</tr>
<tr>
<td class="label">Allosteric modulators</td>
<td>PAMs</td>
</tr>
<tr>
<td class="label">Candidate</td>
<td>Company</td>
</tr>
<tr>
<td class="label">Tanezumab</td>
<td>Pfizer/Eli Lilly</td>
</tr>
<tr>
<td class="label">Fulranumab</td>
<td>Amgen/Janssen</td>
</tr>
<tr>
<td class="label">BIIB074 (gosuranemab)</td>
<td>Biogen</td>
</tr>
<tr>
<td class="label">DV-1178</td>
<td>Domain/Takeda</td>
</tr>
<tr>
<td class="label">Trial</td>
<td>Agent</td>
</tr>
<tr>
<td class="label">NCT00240617</td>
<td>NGF (intracerebroventricular)</td>
</tr>
<tr>
<td class="label">NCT00876824</td>
<td>AAV2-NGF (CERE-110)</td>
</tr>
<tr>
<td class="label">NCT01530529</td>
<td>NGF (intraventricular)</td>
</tr>
<tr>
<td class="label">Various</td>
<td>Small molecule TrkA agonists</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">7 edges</a></td>
</tr>
</table>

NTRK1 (Neurotrophic Tyrosine Kinase Receptor 1) encodes the TrkA (Tropomyosin Receptor Kinase A) receptor, a high-affinity receptor tyrosine kinase for nerve growth factor (NGF)[@barbacid1994]. TrkA is the principal receptor mediating NGF's effects on neuronal survival, differentiation, axonal guidance, and synaptic plasticity. In the adult nervous system, TrkA is expressed primarily in basal forebrain cholinergic neurons, sympathetic neurons, and nociceptive sensory neurons, making it a central player in both neurodegenerative disease and pain disorders[@huang2003].

The NGF/TrkA signaling axis is one of the most extensively studied neurotrophin systems and represents a foundational target for neuroprotective strategies in Alzheimer's disease, where loss of cholinergic innervation is a cardinal feature[@cuello2022]. Dysregulation of TrkA signaling has also been implicated in Parkinson's disease, amyotrophic lateral sclerosis, and peripheral neuropathies.

Protein Information

Structure

Domain Architecture

TrkA possesses a canonical receptor tyrosine kinase structure[@Reichardt2006]:

Extracellular Domain (~320 residues):

• Leucine-rich motifs (LRR1-3): Three leucine-rich repeat segments flanking cysteine-rich clusters
• Two cysteine-rich clusters: Form disulfide-bonded structures mediating ligand-independent dimerization
• Two immunoglobulin-like domains (Ig1, Ig2): Ig2 (C2-type) is the primary NGF binding site; Ig1 may interact with cell adhesion molecules
• The extracellular domain controls ligand specificity — TrkA binds NGF exclusively among neurotrophins

• Single alpha-helical transmembrane segment
• Anchors receptor to plasma membrane
• Transduces ligand-binding conformational change across membrane

• Tyrosine kinase domain: Catalyzes autophosphorylation and substrate phosphorylation
• Activation loop: Regulates kinase activity; phosphorylated on Y670, Y674, Y675 upon activation
• Multiple tyrosine residues: Y496, Y705, Y706, Y751, Y754, Y785, Y791 — each docking site for specific SH2/PTB domain proteins
• C-terminal tail: Regulatory phosphorylation sites and protein interaction motifs

Activation Mechanism

TrkA activation follows a well-characterized sequence:

Normal Function

Neurotrophin Signaling

NGF binding to TrkA activates three major signaling cascades[@Reichardt2006]:

1. Ras/MAPK (RAF/MEK/ERK) Pathway:

• TrkA autophosphorylation recruits Shc and Grb2/SOS
• Ras activation → RAF → MEK → ERK1/2
• Promotes neuronal survival via ERK1/2-mediated transcription
• Drives long-term phenotypic changes including gene expression for differentiation

• Phosphorylated TrkA recruits PI3K via Gab1/2 adaptors
• PIP2 → PIP3 via PI3K; Akt recruited to membrane
• Akt phosphorylates pro-apoptotic proteins (Bad, FOXO, GSK3β)
• Strong anti-apoptotic, pro-survival signaling — the primary survival pathway

• Phosphorylated Y785 recruits PLC-γ1
• PLC-γ1 hydrolyzes PIP2 → DAG + IP3
• DAG activates PKC; IP3 triggers Ca²⁺ release from ER
• PKC activation modulates ion channels, transcription factors, and cytoskeletal dynamics

Biological Roles

Neuronal survival:

• NGF/TrkA is the archetypal survival signal for specific neuronal populations
• Retrograde axonal transport of NGF from targets to cell bodies is required for survival
• TrkA-mediated survival depends primarily on PI3K/Akt signaling

• Guides axonal growth toward NGF-producing target fields
• Promotes phenotypic differentiation of sympathetic and sensory neurons
• Regulates synaptic specificity during development

• TrkA is present at synapses in the adult brain
• NGF/TrkA signaling modulates synaptic strength and plasticity in the basal forebrain
• Cholinergic neuron function — particularly hippocampal and cortical plasticity — depends on TrkA

• TrkA on nociceptive C-fibers mediates inflammatory and heat pain
• NGF/TrkA is upregulated in injured peripheral nerves, driving hyperalgesia
• TrkA antagonists (e.g., tanezumab, fulranumab) are in development for chronic pain

Role in Neurodegeneration

Alzheimer's Disease

The NGF/TrkA axis is central to AD pathogenesis because basal forebrain cholinergic neurons (BFCNs) — which degenerate early in AD — depend on NGF/TrkA signaling for their survival and function[@huang2003][@cuello2022]:

Cholinergic neuron vulnerability:

• BFCNs project to hippocampus and cortex, regions critical for memory
• These neurons synthesize and release acetylcholine, essential for attention and memory encoding
• BFCNs express high levels of TrkA and require retrograde NGF transport from target regions
• Age-related decline in NGF/TrkA signaling contributes to cholinergic dysfunction

• Reduced TrkA receptor expression on cholinergic neurons in aging and AD
• Impaired axonal transport of NGF from cortex to basal forebrain
• Altered NGF protein levels and processing in AD brains
• TrkA signaling becomes uncoupled from downstream survival pathways (PI3K/Akt)

• Exogenous NGF: Delivered intraparenchymally or via gene therapy; showed modest effects on cognition, but significant side effects (pain, weight loss) from off-target NGF effects on sensory neurons[@tuszynski2005]
• TrkA agonists: Small molecule agonists designed to selectively activate TrkA on cholinergic neurons without the sensory side effects[@massardier2022]
• AAV2-NGF gene therapy (CERE-110): Phase 2 trial showed acceptable safety and some signal for cognitive stabilization[@bishop2020]

• NGF does not cross the blood-brain barrier (BBB)
• NGF causes painful hyperalgesia when reaching peripheral sensory neurons
• TrkA agonists must achieve selectivity and brain penetration

Parkinson's Disease

TrkA signaling influences PD through multiple mechanisms:

Dopaminergic neuron support:

• TrkA is expressed in subsets of dopaminergic neurons in the substantia nigra pars compacta
• NGF/TrkA signaling promotes survival of dopaminergic neurons in culture and animal models
• Strategies to enhance TrkA signaling in the nigrostriatal system are under investigation

• NGF is upregulated in Parkinson's disease brains and CSF
• TrkA signaling on glial cells may modulate neuroinflammation
• Anti-inflammatory effects of TrkA activation are being explored

• Co-administration of BDNF (via NTRK2) and NGF (via NTRK1) may provide synergistic neuroprotection
• Gene therapy approaches delivering multiple neurotrophins are in development

Amyotrophic Lateral Sclerosis

• Motor neurons express TrkA in some contexts, particularly during development
• NGF/TrkA signaling may influence motor neuron survival
• Evidence is less robust than for BDNF (NTRK2) in ALS
• Therapeutic targeting remains speculative

Peripheral Neuropathy

This is the most clinically advanced application of TrkA targeting:

• NGF overexpression drives pathological pain and hyperalgesia
• TrkA antagonists (monoclonal antibodies like tanezumab) neutralize NGF, reducing pain
• NGF-neutralizing antibodies have reached phase 3 trials for osteoarthritis, cancer pain, and chronic low back pain
• Side effect of rapidly progressive osteoarthritis (joint destruction) led to FDA hold in 2010-2012, partially lifted subsequently

Therapeutic Approaches

TrkA Agonists (Neuroprotection)

Aiming to activate TrkA signaling for neuronal survival in AD and PD[@massardier2022][@iyerr2023]:

TrkA Antagonists (Pain)

Opposite strategy — blocking NGF/TrkA for analgesic effect[@chen2022]:

Clinical Trials in Neurodegeneration

Mechanism of Action

Cross-Linking Relationships

Related Proteins

• [NGF](/proteins/ngf-protein) — Primary ligand for TrkA
• [BDNF](/proteins/bdnf-protein) — Binds TrkB (NTRK2), not TrkA
• [NTRK2](/proteins/ntrk2-protein) — TrkB receptor for BDNF and NT-4
• [p75NTR](/proteins/p75ntr-protein) — NGF co-receptor (p75 neurotrophin receptor)
• [Shc](/proteins/shc-protein) — Adaptor protein recruited to phosphorylated TrkA
• [PLC-gamma1](/proteins/plc-gamma1-protein) — Effector enzyme downstream of TrkA

Related Pathways

• [Neurotrophin Signaling Pathway](/mechanisms/neurotrophin-signaling) — Core pathway
• [PI3K/Akt Signaling](/mechanisms/pi3k-akt-signaling) — Survival pathway
• [MAPK/ERK Pathway](/mechanisms/mapk-pathway) — Differentiation pathway
• [Cholinergic System](/mechanisms/cholinergic-system) — BFCN survival
• [Axonal Transport](/mechanisms/axonal-transport) — Retrograde NGF/TrkA signaling

Disease Associations

• [Alzheimer's Disease](/diseases/alzheimers-disease) — BFCN survival
• [Parkinson's Disease](/diseases/parkinsons-disease) — Dopaminergic neuron support
• [Peripheral Neuropathy](/diseases/peripheral-neuropathy) — Pain modulation
• [Congenital Insensitivity to Pain](/diseases/congenital-insensitivity-pain) — TrkA loss-of-function

See Also

• [NTRK1 Gene](/genes/ntrk1)
• [NGF Protein](/proteins/ngf-protein)
• [BDNF Protein](/proteins/bdnf-protein)
• [Neurotrophin Signaling Pathway](/mechanisms/neurotrophin-signaling)
• [Alzheimer's Disease: Cholinergic Hypothesis](/mechanisms/cholinergic-hypothesis)
• [Basal Forebrain Cholinergic Neurons](/cell-types/basal-forebrain-cholinergic-neurons)

External Links

• [UniProt: P04629](https://www.uniprot.org/uniprot/P04629)
• [NCBI Gene: NTRK1](https://www.ncbi.nlm.nih.gov/gene/4914)
• [GeneCards: NTRK1](https://www.genecards.org/cgi-bin/carddisp.pl?gene=NTRK1)
• [Human Protein Atlas: NTRK1](https://www.proteinatlas.org/ENSG00000198400-NTRK1)
• [IUPHAR: TrkA receptor](https://www.guidetopharmacology.org/FL8/)

References